A data processing apparatus, such as a microprocessor, may be provided with circuitry to enable it to perform an arithmetic operations on input operands. Typically the circuitry will be capable of performing more than one type of arithmetic operation and the data processing apparatus will be arranged to respond to a number of different program instructions, each corresponding to a specific arithmetic operation, and thus defining both the arithmetic operation to be carried out and the input operands. Generally the result of such arithmetic operations is expected to be determinative, reproducible, and consistent, such that repeated performance of an arithmetic operation can be expected to produce well-defined results.